The treatment of neuromuscular disorders to date has almost universally focused on the development and delivery of therapeutic agents targeting the presynaptic (i.e. neuronal) side of the neuromuscular junction. The various Clostridium toxins in particular have been widely investigated in their native forms as chemodenervation agents. Unfortunately, however, due to the pleiotropic nature of neuronal function the presynaptic therapeutic approach espoused in the art can adversely impact smooth muscle signaling and inter-neuronal signaling in addition to the desired effect on skeletal muscle, thereby creating unwanted side effects and toxicities.
The botulinum toxins in particular demonstrate the problems inherent in presynaptic targeting for treatment of neuromuscular disorders. Over the last ten years commercial preparations of Clostridium botulinum toxins, including BoTox®, have found widespread use as chemodenervation agents for both aesthetic and clinical purposes. Notably, their use in treating symptoms of clinical neuromuscular disorders has recently come under more intense scrutiny by the FDA, due to leakage toxicity resulting in loss of critical smooth muscle cell function. Indeed, even some of the more prevalent toxicities associated with the aesthetic use of these molecules such as persistent dry mouth are again due to leakage of the toxin from the site of administration and inhibition of more distant smooth muscle cell function. In addition, many patients who initially respond to botulinum toxin therapy subsequently become non-responsive to the treatment. Accordingly, for many patients the botulinum injections fail to provide satisfactory long-term treatment of the condition. Nevertheless, despite these apparent drawbacks no alternative therapeutic strategies have been developed to date.
Other neurotoxins isolated from animal venoms are known to have postsynaptic mechanisms of action. For example, α-conopeptides from the venom of Conus marine snails are known to act postsynaptically on nAChRs. The class of snake venom proteins known as alpha-neurotoxins are also known to be competitive antagonists of nAChRs. α-Neurotoxins have a highly conserved fold, due primarily to four invariant disulfide bonds and are classified as either “short” with 60-62 residues and four disulfides, or “long” with 63-80 residues and a fifth disulfide. See Walkinshaw, M. D. et al. Proc. Natl. Acad. Sci. USA 77:2400-04 (1980). All of these toxins bind with high affinity to the muscular-type nAChR, and long chain toxins additionally recognize the alpha7 receptor of the neuronal-type nAChR with high affinity (Servent D. et al.; Eur J Pharmacol. 393(1-3):197-204 (2000).
Long chain α-neurotoxins modified for enhanced binding to the α7-subunit containing neuronal nAChR in particular have been suggested as optimal therapeutic agents for the specific inhibition of neurotransmission. See, e.g., U.S. Pat. No. 6,753,315. Unfortunately, however, this again follows the convention of presynaptic targeting and overlooks the problematic impact of such antagonists on interneuronal signaling and smooth muscle activity. Further, such antagonists can have a detrimental effect on the immune system due to the distribution of the α-7 nAChR. See, e.g., Wang et al., “Nicotinic acetylcholine receptor alpha 7 subunit is an essential regulator of inflammation,” Nature 421:384-8 (2003).
α-conopeptides from the venom of Conus marine snails have been proposed for use in for muscle denervation and paralysis (Olivera et al. U.S. Pat. No. 4,447,356). However, the muscle paralysis achieved by α-conopeptides lasts only fifteen to twenty minutes, due to high dissociation rates and low biostability. For effective muscle denervation treatment a long-term paralytic effect lasting for several hours or days is desirable. Accordingly, α-conopeptides have not been developed as a viable alternative to conventional chemodenervation with botulinum toxin, and there remains a need for an alternative therapeutic approach to treating neuromuscular disorders that can effectively achieve skeletal muscle paralysis while reducing the adverse effects on neuronal and smooth muscle function that are observed with conventional therapeutic neurotoxins.